Ok, I'm completely ignorant on car speakers, so this will probably all sound stupid. I'm putting a three wire speaker into a vehicle with two wires. Presumably the third wire is a tweeter, right? So can I put a resistor in-line with the tweeter so it still plays without blowing? Am I going to be able to tell the difference without it? Would I be better off just leaving it unplugged?

If it's a tweeter, you'd use a capacitor in series with the tweeter. Something in the 1-2uF range should be safe for most any tweeter. Whether it will make a difference depends on a lot of things (source material, tweeter quality, etc...).

Maybe he's talking about one common / ground and two positives / mid and tweet. He could use a crossover to convert the two wires into output for both speakers if the crossover output grounds are shared, otherwise he could just invert the tweeter polarity on both to compensate.

And regards to quality and everything of the speakers, I took them from my parents minivan, but believe it or not they're nicer than my 1992 F150 stock speakers. Unfortunately it is still the stock head unit.

Ummm... I've no doubt it's a ground and 2 positives. That's the only sensible conclusion.
I'm saying it is new to me. I've never seen or heard of such a thing.
It also seems to me that for bi-amping, four wires ought to be better. So is this something designed to be used with an external xover? It almost has to be application-specific.

Alright, sorry for the delays. I was a little off earlier. I actually have 4 wires on the speaker. I don't have the brand on the speaker, other than it's from a Chrysler van. I've also got a picture of the adapter I'm using because that's what fits on the car. So, smart people....go! How do I turn 4 wires into a 2 wire adapter.